Poor performance using discard

[Peter Grandi]

[ ... ]

> /dev/mapper/ssdvg0-testLV on /media/temp type xfs
> (rw,noatime,nodiratime,discard) [ ... ] real 5m7.139s [ ... ]

> There where lockups where the system would pause for about a
> minute during the process.

> ext4 handles this scenerio fine:

> /dev/mapper/ssdvg0-testLV on /media/temp type ext4
> (rw,noatime,nodiratime,discard) [ ... ] real 0m0.943s [ ... ]

> xfs mounted without discard seems to handle this fine:

> /dev/mapper/ssdvg0-testLV on /media/temp type xfs
> (rw,noatime,nodiratime) [ ... ] real 0m1.634s [ ... ]

[ ... ]

> Any suggestions?

* Look at 'vmstat 1' while 'rm' is running.

* Learn how TRIM is specified, and thus why many people prefer
  running periodically 'fstrim' which uses FITRIM to mounting
  with 'discard'.

* Compare with my Crucial M4 flash SSD with XFS:

    #  time sh -c 'sysctl vm/drop_caches=3; rm -r linux-2.6.32; sync'
    vm.drop_caches = 3

    real    0m59.604s
    user    0m0.060s
    sys     0m3.944s

  That's pretty good for ~32k files and ~390MiB. Probably the
  TRIM implementation on the M4 is rather faster than that on
  the Patriot.